1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium by heating the medium having ink applied to its surface layer by a heater device, thereby to fix the ink applied to the surface layer to a fixing layer of the recording medium.
2. Description of the Related Art
An exemplary conventional technique relating to the above field of art is disclosed in Japanese patent application “Kokai” No: Hei. 10-297197. According to this, a metal substrate includes a coloring ground layer acting also as a rust-preventive layer, a transparent resin layer as an optical transparent resin layer formed over the coloring ground layer, the resin layer being made of acrylic resin, polyester resin, urethane resin etc., and an inkjet receiving layer formed over the resin layer and made of e.g. porous alumina. After application of a sublimating ink or pigment on the inkjet receiving layer by an inkjet printing, the sublimating pigment is heated in a heating furnace or by a hot press, whereby the sublimating pigment in the inkjet receiving layer is sublimed into the transparent resin layer. Then, the inkjet receiving layer is removed to obtain an ornamental metal body having a colored pattern fixedly formed within the transparent resin layer.
According to further art disclosed by Japanese patent application “Kokai” No: 2001-105638, sublimating ink is transferred from an ink ribbon onto a surface of a recording sheet. In order to heat and fix the ink on the sheet, the sheet is charged into a heater box, in which the sheet is advanced and heated between a press roll and a heat roll opposed to each other with a small gap therebetween or between a heat roll and a conveyer belt disposed along a portion of the peripheral face of the heat roll, and then the sheet is discharged from the heater box immediately.
Further, in the field of textile printing, according to an exemplary technique disclosed by Japanese patent application “Kokai” No: Hei. 08-311782, dye is applied to a textile by the inkjet printing method. Then, in order to reinforce the fixing of the dye and also to improve its color development, the textile is charged into a heater device to be heated therein. Then, the textile is discharged from the device immediately to be cooled at the room temperature.
Still further, Japanese patent application “Kokai” No: Hei. 10-16188 discloses an image forming apparatus. According to this, first, a primary image is formed on a thermal transfer sheet by e.g. an inkjet printer. Then, this thermal transfer sheet having the image formed thereon is laid over a recording sheet and these sheets are pressed and heated together, whereby the image (ink) formed on the thermal transfer sheet will be sublimed by the heat and transferred onto an ink fixing layer of the recording sheet, thus forming a secondary image thereon. With this, a finished printed product is obtained.
Another image forming apparatus is known from Japanese patent application “Kokai” No: Hei. 10-230589. According to this, a laminated material layer is provided in advance on an ink fixing layer of a recording sheet. Then, an image is formed on the laminated material layer by e.g. an inkjet printer. Then, the resultant sheet is pressed and heated by heat rolls, thereby to make the laminated material layer transparent and also to fix the ink pigment on the fixing layer. With this, a finished printed product is obtained.
With these image forming apparatuses, sublimating ink is discharged against the recording medium which usually is being transported along a sub-scanning direction, so that an image is formed thereon with ink droplets (here, these will be referred to as “un-sublimated print dots”). Then, during the subsequent heat fixing process, these ink droplets are heated to sublime, so that the sublimed ink pigment (referred to here as “sublimed print dots”) is fixed in the fixing layer of the recording medium, whereby a final printed image formed of the sublimed print dots is obtained.
For this reason, the heating behavior of the ink in the fixing layer during the above heat sublimating process is crucial as this provides determinant effect on the sublimation fixing characteristics, consequently significantly affecting the quality of the printed product as the finished product. This heating behavior, that is, the sublimation fixing characteristics, depends on such factors as the type of ink and/or of the recording medium used and the specific mode of heat sublimating method employed. Referring to one example, according to a finding of the present inventors, there is observed reducing tendency in the density of the final image (sublimated print dots) in case the heating of the recording medium by the heater device during the heat sublimation process is insufficient in terms of the duration and/or temperature of the heating. Conversely, with excessive heating, there is observed occurrence of ink bleeding, which results in disadvantageous reduction in the sharpness of the image. According to one reasonable explanation for this, insufficient heating causing insufficient sublimation of the ink droplets provides sublimated ink dots of insufficient density, whereas over-sublimation of the ink due to excessive heating results in significant diffusion of the ink pigment, producing blurred sublimed print dots.